1. Field of the Invention
The present invention relates to polyimide-based solutions which can be directly spun and shaped.
It also relates to a process for producing these solutions and to a process for the spinning thereof.
2. Background Discussion
The preparation of polyimides from diisocyanates and acids or anhydrides has been known for a long time.
Thus, FR 2,107,998 describes the preparation of polyimides resulting from 3,3',4,4'-benzophenonetetracarboxylic acid dianhydride and diisocyanates. However, such a reaction requires an extremely long reaction time which makes this process expensive for industrial application.
To reduce the reaction time between diisocyanates and dianhydrides, while producing polymers with a sufficient molecular weight to make possible their conversion, and soluble in the usual solvents according to U.S. Pat. No. 4,001,186, the polycondensation has been carried out in the presence of catalysts such as alkali metal methoxides.
EP 0,235,388 also recommends the use of a catalyst, alkali metal fluoride, optionally in combination with a quaternary onium salt, to produce gel-free polyimides.
However, the presence of catalysts is not a favourable component in the production of polyimides, because, in order to produce shaped articles, in particular yarns having good characteristics, the catalyst must be removed before conversion or spinning and this removal is difficult to carry out straightforwardly. Furthermore, for costs reasons, recycling of the catalyst is necessary industrially.